Optical feedback photometer



June '15, 1954 M. H. SWEET 2,680,990

OPTICAL FEEDBACK PHOTOMETER ori inal Filed April 4, 1946 25 l4 1.: /a /ol2 20 "I LINEAR 7 AMPLIFIER 2/ /5 I9 INVENi'OR MONROE H. smrsr ATTORN EYS UNITED 2,680,990 OPTICAL FEEDBACK PHOTOMETER Monroe H. Sweet,Binghamton, N. Y., assignor to General Aniline & Film Corporation, NewYork, N. Y., a corporation of Delaware April 4, 1946, Serial No.

.. OFFICE Original application 659,457, now Patent No. 2,649,834, datedAugust 25, 1953. Divided and this application December 28, 1951, Serial41 Claims.

This invention relates to photoelectric measuring circuits, and moreparticularly to circuits for apparatus used in determining photographicdensity.

In numerous mensuration operations, an object is to obtain a directindication of values which are logarithmic functions of the quantity. Atypical example is the measurement of the density of a photographicfilm. In the usual measuring instrument for determining the value of thefilm density, the light flux transmitted through the film is directedupon a suitable photo-sensitive device, such as a photo-emissive vacuumtube. The output of the phototube is then amplified and the relativevalue thereof indicated upon a suitable meter. The radiant fiux incidentupon the phototube is an inverse logarithmic function of the density ofthe film. If the output of the phototube is linearly amplified, theindicating meter must be provided with a scale which is graduatedlogarithmically, in order to read density directly on. the meter. As isknown, a logarithmic scale is non-uniformly graduated, the indiciathereof being crowded near one end of the scale and being Widely spacednear 1.

the opposite end thereof. The non-uniformity of the scale graduationsadversely affects the accuracy and facility of the meter readings.

Various expedients have been proposed for obtaining direct indicationsof density on a meter having a substantially uniformly graduated scale.Among other expedients, cut pole pieces have been used in the meter tovary the sensitivity of response thereof over different portions of thescale. The results obtainable by such expedients have been generallyunsatisfactory. In my U. S. Patent 2,406,716 entitled Direct ReadingDensitometer, there is described a suitable electronic measuring circuitfor obtaining direct readings upon a uniformly graduated meter scale ofthe density of a photographic film. In this circuit, a logarithmicamplifier is provided between the output of a phototube and theindicating meter. The parameters of the amplifier are so selected thatthe meter indicates directly the density of the sample upon a uniformlygraduated scale. The light flux incident upon the phototube is aninverse logarithmic function of the density of the sample, and theoutput current of the phototube is a direct function of the lightincident thereupon. By interposing a logarithmically re sponsiveamplifier between the phototube and the meter, such compensation iseffected that the relative current flowing through the meter becomes adirect function of the densityof the samd pie. The patented circuit hashad very satis-- factory commercial use.

Instead of a logarithmic amplifier, it is in many respects desirable toemploy a conventional linear amplifier which is simpler to construct andmaintain in operation. Accordingly, the present invention comprises ameasuring circuit in which, although a linear amplifier is used, directindication is obtained of logarithmic values. The necessary compensationis effected by controlling the luminous excitation of the light sourcein accordance with the response of a photo-emissive vacuum tube orsimilar photo-sensitive element.

It is, therefore, among the objects of this invention to provide asimple, logarithmically responsive measuring circuit; to provide such acircuit including a linear amplifier controlling the intensity of alight energy emitting element responsive to the output of an energysensing element, the latter receiving incident light energy from theenergy emitting element; to provide a logarithmically responsivemeasuring circuit including a light source, a photo-emissive vacuum tubearranged in operative relation therewith, and a linear amplifiercontrolling the energizing of the light source in response to the outputof the photo-emissive vacuum tube; and to provide such a logarithmicallyresponsive measuring circuit in which the indicating means may beconnected either in the output circuit of the amplifier or in the inputcircuit of the light source.

These and other objects, advantages and novel features will be apparentfrom the following description of the invention pointed out inparticularity in the appended claims and taken in connection with theaccompanying drawing, in which:

Figure 1 is a block diagram illustrating the general interconnection ofthe component elements;

Figure 2 is a schematic circuit diagram of one embodiment of theinvention; and

Figure 3 is a schematic circuit diagram of another embodiment of theinvention.

Basically, the measuring system, in accordance with the presentinvention, comprises a radiant flux receiving element such as aphototube arranged to receive light from a source through a translucentsample. The phototube current, which is directly proportional to thelight intensity, is amplified by means of a conventional linearamplifier, and the output of the amplifier in turn is utilized tocontrol the luminous excitation of the source. In this manner a feedbackloop is established between the linearly amplified output of thephototube and the source from which excitation of the light is derived.

The salient feature, in the above circuit arrangement, is that, althoughlinear response con ditions exist throughout the system, alogarithmically varying quantity, such as the density of a sample, maybe directly indicated over a scale of an indicating meter which isuniformly graduated in terms of density. This is accomplished byemploying a lamp for the light source which converts electrical energyinto radiant energy and has such inherent characteristics that theradiant energy bears a logarithmic relation to the electrical energysupplied to the lamp; in other words, the candle power output thereofvaries approximately logarithmically as a function of the electriccurrent or voltage applied to the lamp. Various types of incandescentlamps are known to possess this desirable characteristic.

It was mentioned before that in determining the density, the radiantflux incident upon the phototube is an inverse logarithmic function ofthe density of the film. Consequently, when this circuit is used fordensity measurements, the amount of light the phototube from the lampthrough the sample varies inversely with the logarithm of the density.i'he measuring instrument connected directly to indicate the phototubecurrent would require a logarithmic scale unless provision is made forchanging this indication, that is, for correcting the indication inaccordance with a logarithmic compensator. Intead of varying theindication of the meter anywhere in it energizing circuit the lightintensity which excites the phototube is caused to vary in such manneras to compensate for the linear response of the phototube and provide alogarithmic response in the system. With this compensation in thecircuit response, the indicating meter may have a standard, uniformlygraduated scale to give direct readings of the density of the sampleinterposed between the lamp and the phototube.

Referring to Figure 1 of the drawing, the invention is illustrated, byway of example, as incorporated in a direct reading densitorneter inwhich a uniformly graduated scale meter provides direct indication ofthe density of a sample being examined. For this purpose, light from asuitable electric iamp iii, which is an incandescent lamp, isconcentrated by a condenser lens ii and directed, a suitable filter l2and a sample i3 mounted on a support Hi, upon the cathode H of aphotoemissive vacuum tube l5. Cathode E5 and anode ii are connected byconductors l3 and i9, respectively, to the input of a linear amplifier2t. Amplifier 2c is supplied with operating potential from a suitablesource of preferably constant D. C. potential, not shown here, connectedto terminals 2!. The amplifier includes means for controlling theillumination of lamp it, in a manner described more fully hereinafter,and for this purpose, the lamp H3 is shown connected to the amplifier bymeans of conductors 2'2. A suitable indicating meter 25 is actuated bythe amplifier 2t, and is shown connected thereto as indicated.

The operation of the arrangement is as follows. The density of sample[-3 is an inverse logarithmic function of its light transmission. Thetransmission, in turn, is a measure of the amount of light reachingphototube is from source it! through the sample with a constantintensity of the source. From the data relating lamp current or voltageapplied to its terminal to relative lamp candle power, it can be shownthat the candle power is a logarithmic function of the applied voltage,insofar as respects an incandescent iamp (see Weaver and Hussong, Noteon the Color Temperature-Candle Power Characteristics of Tungsten Lamps,J. O. S. A. 29, Jan. 17, 1939). Accordingly, if the operating potentialapplied to light source it! is modified as an inverse function of theoutnot current of phototube it, the candle power of source II) will bevaried logarithmically as an inverse function of the phototube outputcurrent. Consequently, a suitable current or voltage indicating meansconnected either in the output circuit of phototube if; or in theenergizing circuit of source it will directly indicate the density on auniformly graduated scale.

In efiect, the intensity of lamp it is controlled in such a manner thatat low densities (i. e., high light transmission values) the light fluxincident upon phototube iii is a minimum, and as the density of sampleit increases (i. e.,, less light is transmitted) the intensity of lampH3 is also increased. With such an arrangement, it is possible to obtaina uniform response of meter 25 to the density of sample i3.

If meter 25 is calibrated to read densities from 0.0 to 3.0, which is auseful range, and with high linear amplification of the output currentof phototube t5, the following relationships apply:

where F0 is the lamp intensity as automatically corrected, or modulated,to obtain direct readings of density upon the substantially uniformlygraduated scale of meter 25, M is the relative meter response and Fe isthe relative light flux which would be transmitted if the lamp intensitywere not changed. Furthermore,

Fo=antilog (3.0-D)

where D is the density of sample l3 and unit flux is assumed for thelamp at density 3.0. From these two equations, it will be apparent thatantilog (3.0- D) In other words, in order to obtain uniform meterresponse for differing density values of sample 13, the corrected lampintensity must vary directly as the meter response and inversely as theantilog of the total meter density reading minus the density of samplel3. As explained above, this is accomplished in the present circuit bylinear compensation of the operating potential applied to light sourceHl, which results in logarithmic variation of the candle power or fluxoutput thereof.

Figure 2 represents a specific embodiment of an arrangement forobtaining the results of the circuit of Figure 1. The operatingpotentials are derived from a suitable source of substantially constantD. 0. potential connected between terminals 2| of a voltage divider 3G.Cathode l6 of phototube I5 is connected through conductor 18 to a tap 25on the divider 30. Conductor l9 connects anode if to the control grid 21of an amplifier tube 35. Cathode 28 of tube 35 is connected to a tap 3!of divider 30. Anode 32 of tube 35 is connected through a suitable loadresistor 33 and in series with the indicating meter 25 to tap 26.

The load resistor 34 of the phototube I5 is also utilized as the inputcircuit or grid resistance for the amplifier tube 35. A minimum biasvoltage for this tube is derived from the voltage divider betweencathode 28 and tap 36. The ohmic value of resistor 34 is preferably highin the neighborhood of megohms in order to insure a satisfactory voltagedrop for biasing the amplifier tube 35 when the phototube is conducting.

With the described arrangement, as the light falling upon phototube 15from source I 0 through sample 13 increases, the phototube outputcurrent increases. This current produces a voltage drop across resistor34 driving the grid 2'! more positive, which, in turn, increases theconductivity of tube 35. The increase or decrease in the conductivity oftube 35 is utilized to change the effective bias voltage of a controltube 50. The purpose of this arrangement will be apparent hereinafter.

The incandescent lamp [0 is energized from an alternating current sourceconnected to terminals 31 and applied to the primary winding 38 of atransformer 40. The secondary winding 4! of transformer 40 is connectedin series with the primary winding 42 of transformer and control tube56. Conductors 2?. connect the secondary winding 43 of transformer 55 tothe filament of lamp Ill. In a specific example, transformer 40 mayraise the potential of the supply line to 500 volts in order to providesufiicient anode voltage for the control tube 50. Transformer 45, on theother hand, will reduce the voltage applied to the primary winding 32 toabout six volts required for the lamp I 0 which may have a six candlepower rating.

The secondary winding 4! of transformer 40 and the primary winding :2 oftransformer 45 are in series and the free terminals thereof connect tothe anode it and cathode 44 of the control tube 59, respectively. Thecircuit of the control grid 5i is completed to the cathode 44 throughthe anode load resistance 33 of the amplifier tube 35 in series with theindicating 25.

As can be seen from the circuit, the eifective anode voltage for thetube is derived from the secondary winding 4| of the transformer 4i],whereas the grid voltage is derived from the current flowing through theload resistor 33. When there is no current flowing in this resistor, thegrid 5! has zero bias and is effectively at cathode potential. Thiscondition calls for maximum anode current of tube 59 providing a maximumenergy transfer from the A. C. source to the exciter light it. Anyvariation in conductivity of the tube 35 will cause correspondingcurrent variations in the load resistor 33 resulting in a correspondinggrid voltage change for the tube 50. This voltage is negative withrespect to the cathode 44 inasmuch as the conductivity of the amplifiertube 35 impresses on the grid 5! a potential more negative than thecathode 44.

Current variations of the tube 35 as pointed out above depend on thelight intensity reaching the photocell and are a direct functionthereof. These variations impressed on the control tube 5! will causevariations in the light intensity of the exciter lamp H] inasmuch as thecurrent in the primary winding 42 depends on the conductivity of thetube 50. In view of the fact that the characteristics of the exciterlamp I 9 are such that variations of filament excitation produce alogarithmic variation of candle power meter output the light energyimpressed on the photocell will vary logarithmically with respect to thelinear variation of energizing voltage of the lamp it.

As set forth above, the anode potential of tube 35 varies inversely withthe output current of phototube l5, and thus inversely as a linearfunction of the light incident upon the latter from lamp l0 throughsample I3. Accordingly, the illumination of lamp i0 is varied as aninverse logarithmic functon of the light incident upon phototube I5.Meter 25 measures the cur rent flow in the output circuit of amplifier35, and thus indicates directly, on a uniformly graduated scale, thedensity of sample I3 as measured by phototube [5, in accordance with therelations between compensated lamp intensity and incident flux versusmeter reading set forth above.

Figure 3 represents another embodiment of the invention which difiersfrom that shown in Figure 2 only in the location of meter 25. In Figure3, elements identical with those in Figure 2 have been given the samereference characters. As shown, meter 25 is connected in the supplycircuit of lamp l0, being serially interposed between conductor 22 andone terminal of winding 43. As the energizing voltage of lamp I0 varieswith the conductivity of control tube 50 which is dependent on the anodevoltage of tube 35, the same relation exists as in the circuit of Figure2 and the current indication of meter 25 dirctly represents the densityof sample [3 upon a uniformly graduated scale. The circuit of Figure 3otherwise operates in the same manner as does the circuit of Figure 2.

With the described arrangements, an optical feedback effect is producedin which the energizing of the light source is varied as an inversefunction of the phototube output current. This, in turn, varies the lampintensity as an inverse logarithmic function of the phototube outputcurrent. Consequently, logarithmic compensation necessary in densitymeasurements is automatically accomplished so that a meter whoseresponse is linear with respect to current may be used to directlyindicate density upon a substantially uniformly graduated scale. Thus, alogarithmically responsive measuring circuit is provided incorporating alinear noncompensated amplifier which need not be operated under suchconditions as to have a logarithmic relation of grid potential to gridcurrent.

It will be understood that other means of controlling the energizingcurrent of the light source in response to the output of the phototubemay be used. For example, a radio frequency oscillator circuit, such ascommonly used in sound motion picture projectors for the exciter lamps,may be incorporated in the present arrangement. The essentialrequirement is that the intensity of the light source decreases inresponse to an increase in the flux incident upon the photo element, andthat the overall gain of the amplifier system is high.

This application is a division of my copending application, Serial N 0.659,457, filed April 4, 1946, for Optical Feedback Photometers, nowPatent No. 2,649,834, issued August 25, 1953.

I claim:

1. An electrical measuring apparatus for directly indicating thephotographic density of translucent sample materials, comprising aphotoelectric tube, a source of operating potentials .tronic means itherefor, an exciter lamp arranged to illuminate said tube throughsamples which may be placed between said lamp and said tube, said lambeing of the filamentary incandescent type having such characteristicsthat the light intensity emitted therefrom bears a logarithmic relationto the electrical current supplied to the filament, a circuit forsupplying current to said filament, elecof variable conductivity in saidcircuit for varying the magnitude of said filament current, a controlcircuit including electronic control, means connected between saidphototube and said filament current varying means operable to alter theconductivity of said current varying 'means in inverse relation to thephototube current whichv is directly related to the light reaching saidphototube upon a sample being placed between said lamp and saidphototuoe, whereby the light intensity of said lamp is varied in inverserelation to the density of said sample, and

av current fiow measuring means in said control circuit, the indicationof which may be marked directly in linearly spaced density values.

2. An electrical measuring system for determining the density oftranslucent materials, comprising a photoelectric tube, a loadresistance and a source of operating potential for said tube, an exciterlight arranged to illuminate said phototube through said translucentmaterials, said exciter light comprising a filamentary incandescent lamphaving such characteristics that the light intensity emitted therefrombears a logarithmic relation to the electrical energy supplied to saidfilament, a circuit for energizing said filament including a. source ofpotential and a vacuum tube having anode and cathode electrodesconnected efiectively between said source and said filament and a gridelectrode connected to said cathode through a grid resistor, theconductivity of said vacuum tube determining the current supplied tosaid filament and control means comprising an amplifier tube having aninput circuit including said load resistance and an output circuitincluding said grid resistor, whereby current variations in said outputcircuit control the conductivity of said vacuum tube in inverse relationto the response of said phototube, and the light intensity of said lampis varied in inverse relation to the density of said material and acurrent indicating device connected in said output circuit, theindication of which may be marked directly in linearly spaced values ofdensity.

3. An electrical measuring system for determining the density oftranslucent materials, comprising a photoelectric tube, a loadresistance and a source of operating potential for said tube, an exciterlight arranged to illuminate said phototube through said translucentmaterials, said exciter light comprising a filamentary incendescent lamphaving such characteristics that the light intensity emitted therefrombears a logarithmic relation to the electrical energy supplied to saidfilament, a circuit for energizing said filament including a source ofpotential and a vacuum tube having anode and cathode electrodesconnected effectively between said source and said filament and a gridelectrode connected to said cathode through a grid resistor, theconductivity of said vacuum tube determining the current supplied tosaid filament and control means comprising an amplifier tube havinganode, cathode and control electrodes, an input circuit including saidload resistance between said grid and cathode electrcdes and an outputcircuit including said grid resistor and an anode current meter betweensaid anode and cathode electrodes whereby anode current variations insaid output circuit develops a voltage drop across said grid resisterand control the conductivity of said vacuum tube in inverse relation tothe response of said phototube, and the light intensity of said lamp isvaried in inverse relation to the density of said material, theindication of said meter being proportional to the density of saidmaterial may be marked directly in linearly spaced values.

4. An electrical measuring system for determining the photographicdensity of translucent materials, comprising a photoelectric tube, aload resistance and a source of operating potential for said tube, anexciter light arranged to illuminate said phototube through sampleswhich may be placed between said lamp and said tube, said exciter lightcomprising a filamentary incandescent lamp having such characteristicsthat the light intensity emitted therefrom bears a logarithmic relationto the electrical energy supplied to said filament, a circuit forenergizing said filament including a source of potential and a vacuumtube having anode and cathode electrodes connected effectively betweensaid source and said filament and a grid electrode connected to saidcathode through a grid resistor, the conductivity of said vacuum tubedetermining the current supplied to said filament and control meanscomprising an amplifier tube having anode, cathode and controlelectrodes, an input circuit including said phototube load resistancebetween said grid and cathode electrodes and an output circuit includingsaid grid resistor and an anode current meter between said anode andcathode electrodes whereby anode current variations in said outputcircuit develop a voltage drop across said grid resistor, theconductivity of said vacuum tube being thereby controlled in inverserelation to the current conductivity of said phototube and the lightintensity of said lamp being varied in inverse relation to the densityof said material, the indication of said meter being proportional to thedensity or" said material, may be marked directly in linearly spacedvalues.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,973,469 Denis Sept. 11, 1934 2,241,557 Nichols May 13, 19412,245,124 Bonn June 10, l9i1

